Usually, a transmitter or receiver uses FSK (Frequency Shift Keying) modulation for the short distance transmission or reception of data or commands. If the RF carrier frequency is high, for example more than 300 MHz, quite a high bandwidth is selected for the intermediate frequency, particularly more than or equal to 100 kHz. The modulation frequency shift in the modulated signals can be adapted in accordance with this bandwidth. In such case, a reference frequency can be used, supplied by a local oscillator, which is not very precise and thus inexpensive. However, account has to be taken of thermal noise power, which is proportional to the selected bandwidth. Thus, a broadband transmission or reception system generally has not excellent sensitivity.
FR Patent No. 2 711 293 discloses a radio-frequency signal receiver, whose bandwidth can be adapted. This superheterodyne type receiver is capable of demodulating both broadband frequency modulated signals, and narrow band signals. The bandwidth can be varied dynamically to obtain the best compromise between attenuation of distortion and attenuation of noise. After a first frequency conversion, a filter filters the intermediate signals with a first bandwidth (broadband). Next, another filtering operation is performed on the intermediate signals using a second bandwidth and a third bandwidth of filter for a narrow band. Finally, a discriminator performs demodulation.
However, no specific receiver configuration is performed in the receiver in accordance with the modulated data rate of the incoming data signals. Consequently, the dynamic bandwidth variation provides a good compromise between attenuation of distortion and attenuation of noise for the incoming signals. The structure of the receiver of this Patent document is also relatively complicated, which means that its electric power consumption cannot be sufficiently reduced for it to be mounted in a portable object of small size.